This invention relates to an improved process for making a granular fertilizer product having enhanced strength and suitable size and weight for mechanical dispensing and application to and into the soil. The basic process of making the granular fertilizer product is disclosed in the commonly assigned application of Allan et al., U.S. Ser. No. 305,603 filed on Sept. 25, 1981, the disclosure of which is expressly incorporated herein by reference. These granules combine fine melamine crystals with urea, binding the fine malamine crystals into a granular form.
There is a continuing search and need for improved fertilizer materials. For example, while ammonium nitrate, containing 34% N, still ranks second only to ammonia, 82% N, as a source of fertilizer nitrogen, its use generally has been decreasing in terms of market percentage, since 1965. The reason is increased use of the higher nitrogen content materials, ammonia, with 82% N, and urea, with 46% N, respectively. The use of urea is a development of recent years, and may have been prompted in part by a desire to reduce shipping costs.
All of these nitrogen fertilizer materials; ammonia, ammonium nitrate and urea, are readily soluble in water. They are therefore subject to leaching, and their use results in a rapid release of their nitrogen. Since this necessitates repeated applications for sustained growth, or one application with higher leaching losses, there have been many developments relating to slow release nitrogen fertilizer materials. Generally such materials sacrifice nitrogen content for some degree of control over nitrogen availability.
Melamine and its hydrolysis products, ammeline, ammelide, and cyanuric acid, have often been considered as potential sources of nitrogen for incorporation in fertilizer compositions or for utilization as nitrogen sources per se. Melamine with 66.6% N would provide substantial amounts of nitrogen. However, at present it is more expensive than urea. Moreover, commercially produced melamine is available only as a fine crystalline powder. It is manufactured in the form of very fine crystals because small size particles are required for the present commercial end markets for melamine, such as, for example, the production of melamine-formaldehyde resins and the production of fire retardant paints.
A typical screen analysis for one commercially available melamine, conducted with United States Standard Sieve screens, is as follows:
______________________________________ Screen Analysis Percent Retained ______________________________________ 40 mesh 0-0.1 40-50 mesh 0-0.1 50-60 mesh 0-0.3 60-80 mesh 0.5-5.0 80-100 mesh 1.0-5.0 100-200 mesh 13-30 300-325 mesh 13-30 Through 325 mesh 40-60 ______________________________________
The commercially-produced small melamine crystals are desired by the resin producers because the small crystals dissolve more readily, and any larger particles, if present, would tend to require a longer processing time; therefore, the larger particles are less desirable. In the fire retardant paint market, the melamine crystals are dispersed in the paint, where the current fine particle sizes produce a smoother texture in the dried paint than would larger particles. The fine particle sizes of the commercially available melamine products make melamine a product that is not very attractive for agricultural applications.
Moreover, the fine particle sizes of commercial melamine as currently produced, make it impractical to use as a fertilizer material. The fine particles, if applied to the surface of the ground, would be blown away by even mild winds. If applied by air, as from an airplane or helicopter, drifting would be a serious problem and would cause uneven application. If applied through mechanical applicators, the fine particles would tend to form bridges and thus would plug transfer and dispensing lines. These difficulties in handling the commercially available melamine solids would make any large scale agricultural application impractical.
Past investigations into the possible use of melamine as a fertilizer nitrogen have generally been on a small scale where the limitations imposed by the fine particle size of melamine were not a serious obstacle. In most cases negative results were obtained. Because of these reported negative or equivocal results, little work has been done that has been concerned with the physical forms of melamine. On the contrary, past work has addressed the question of whether melamine and its salts are in fact useful sources of nitrogen and in general, the most frequent conclusion reached was that they were not. Consequently, apparently no one to date has taken the further step of trying to place these materials in a phyical form that would seek to optimize their performance if used.
Wolf, in West German Pat. No. 926,853 in 1954, suggested the use of the melamine-formaldehyde resins as binding agents for granular fertilizer compositions also containing spent sulfite liquor and super phosphate. No field trials were reported in the patent.
In 1964, Hauck and Stephenson published an article in Agricultural and Food Chemistry 12, 147-151, describing the rate at which symmetrical triazines converted in the soil to a form useful to plants. For evaluation, melamine phosphate and melamine nitrate were recrystallized, respectively, washed, and dried. What the authors refer to as granules of melamine, acid, and metal ion were prepared by forming dried pastes of the several materials, then crushing and screening the product to pass -8+12 mesh. Such materials included mixtures, for example, of melamine and phosphoric acid, melamine and nitric acid, and melamine and ferric ammonium sulfate. In addition, the performances of melamine, ammeline, ammelide, and cyanuric acid were evaluated in silty clay loam in practice form while melamine and cyanuric acid were also evaluated as solutions. Although some degradation of all was observed, the authors pointed out that melamine and cyanuric acid powders degraded at a faster rate than either granules having sizes in the range from -8+12 mesh, or solutions. The authors concluded with what appears to be a very negative observation, that the evaluation of these materials as slow-release nitrogen sources should be made only on crops that are expected to respond to small amounts of nitrogen added at frequent intervals.
In 1976, East German Pat. No. 120,645 to Wolf described the use of a polymer coating on prilled urea to provide a slow-acting fertilizer. Prills having an average particle diameter of 1.7 mm were pretreated with a coating substance or solution, dried, fluidized, and then coated with a latex polymer at 60.degree. C. Melamine was mentioned as one of the possible pretreating substances. The coated pellets had particle sizes in the range from 0.5-5 mm, preferably in the 1-3 mm range.
U.S. Pat. No. 3,705,019 describes the production of granular cyanuric acid from fine cyanuric acid powder particles, to produce fast dissolving granules for treating the water in swimming pools. It has nothing to do with fertilizer.
Subsequently, Corte et al. in U.S. Pat. No. 4,083,712 produced nitrogenous fertilizers in the form of salts of a cation exchange resin. These fertilizers were in the form of beads, granulates or powders. In Example 3 of the patent, a sulfonated polystyrene cross-linked with divinyl benzene, in the hydrogen form and strongly acidic, was reacted with an aqueous suspension of melamine. The reaction product was said to consist of 100 ml of a nitrogenous fertilizer containing 2.2 moles of melamine per liter. This material, and other ion exchange resin salts produced from guanidine and other nitrogen compounds, were tested over a two year period with grass in pots of loamy sandy soil which had received a basic dressing of phosphorus and potash.
In what appears to be a subsequent development, described in South African Pat. No. 735,583, Corte and his associates pursued their investigations further. More work is described with nitrogenous fertilizer salt compositions comprising a cation exchange resin having a nitrogen base such as melamine chemically bound thereto. The cation exchanger optionally may be partially charged with a material such as melamine, and partially with inorganic nutrient ions.
Both of these patents of Corte et al. point out that certain slow release nitrogen sources, such as urea-formaldehyde resins, have been combined with a short term nitrogen fertilizer material, such as a nitrate, to provide a greater initial fertilizing effect. Corte et al. observed, however, that melamine, unless chemically reacted to become the salt of an ion exchange resin, was "unsuitable for fertilizing purposes", U.S. Pat. No. 4,083,712, col. 2, lines 58-61.
The commonly assigned application Ser. No. 305,603 describes the composition and process of making of a melamine urea granule or prill. However, none of these authors described the annealed high strength melamine-urea granules of the present invention.